Generally kids wheels have a lot less spokes than a 26" or a 700c. And the rims are definatly not held to as strict of standards. So its alot harder to perfectly dial in a wheel. As mentioned above, tension is a big issue. Is the wheel is round and just not straight, You may need to take the tire off and "adjust" the rim itself with a couple well places "taps" of a hammer... If its not round and seems like the hub isnt centered in the rim, then the tension may be off.

Hope this helps.